Rolling of soft microbots with tunable traction

Microbot (μbot)–based targeted drug delivery has attracted increasing attention due to its potential for avoiding side effects associated with systemic delivery. To date, most μbots are rigid. When rolling on surfaces, they exhibit substantial slip due to the liquid lubrication layer. Here, we introduce magnetically controlled soft rollers based on Pickering emulsions that, because of their intrinsic deformability, fundamentally change the nature of the lubrication layer and roll like deflated tires. With a large contact area between μbot and wall, soft μbots exhibit tractions higher than their rigid counterparts, results that we support with both theory and simulation. Upon changing the external field, surface particles can be reconfigured, strongly influencing both the translation speed and traction. These μbots can also be destabilized upon pH changes and used to deliver their contents to a desired location, overcoming the limitations of low translation efficiency and drug loading capacity associated with rigid structures.


Supplementary Text
Section S1. Estimation of the gap between a rigid Pickering sphere and the glass substrate The gap between a rigid Pickering sphere and glass wall h was calculated with a balance between gravitational Fg and electrostatic forces Fe (46). The gravitational force Fg is where R is the sphere radius, g is the gravitational constant, is the effective density of the Pickering droplet, and is the solvent density. The electrostatic force is where #" is the Debye length, h is the separation between the sphere and the glass substrate, is the vacuum permittivity, is the dielectric constant of the solvent, is Boltzmann's constant, and ! " and ! are the surface potentials of the decylamine-modified beads and the glass slide, respectively (47). A plot of ln(h/R) vs. R is shown in Fig. S4, which reveals a scaling Since the droplet is force-free, the balance of pressure and shear forces yields a simple relationship between the droplet's translation velocity U and the tank-treading velocity of beads on the surface Vs In the limit of N ≫ ℎ , 6 − 7 = 6 − 8 9 = 2B/ℎ and, when q is small, U ~ ωpR, the traction approaches one.
Section S3. Calculation of the surface particle energies in different configurations (S10) where 〈·〉 indicates time-averaging. In this, ^a b is the magnetic dipolar interaction between a pair of paramagnetic particles with mutual interaction described by (48) where } = 4 f ji ~ is the induced dipole moment on one particle and h = |}|, rij = rjri is the position vector between particles i and j, f = •€ •‚ •, and g is the angle between the vector rij and the magnetic field H, ƒ"…g = . For a 3D precessing magnetic field of the form ~= ‡ 3: cos 8 ‰ Š ‹ OE + ‡ • Ž OE + ‡ 3: sin 8 ‰ Š • OE, the time-averaged pair interaction can be obtained by integrating and normalizing Eij over one period 2π/ωM.
where f • and f : are the y-and z-component ofab , respectively.
To place the particles at the droplet interface, we consider dense packing of surface particles either at the pole or the equator with the coordinates obtained by the icosahedron mesh method. Here, the sphere surface was separated into uniform isosceles triangles and a geosphere mesh was created with the software 3ds Max. The separation between neighboring particles was chosen as 1.1 µm with a local packing of 0.9. With identified particle coordinates for a specific configuration, we calculated the total energy of the system by summing all pair interactions based on Eq. (S12).       Because the droplet is lighter than medium, it floats to the top and contacts with the textured surface.

Figure S1. FTIR of magnetic particles before (black line) and after (red line) surface modification with decylamine.
Movie S10. Pickering emulsion droplets retain 95% stability in 1× PBS buffer solution.
However, droplets become unstable and burst after adding a base solution to reach pH 12.